Isomerism – Isomers

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What is isomerism ?

Different compounds having same molecular formula but different properties are called isomers. The phenomenon of existence of such compounds is called isomerism. Eg.

isomers examples

Types of isomerism

Isomerism can be divided into two broad classes:

  1. Structural isomerism.
  2. Stereoisomerism.

types of isomerism

Structural isomerism

Compounds having same molecular formula but different structural formula are called structural isomers. Phenomenon of existence of such compounds is called structural isomerism.

Structural isomerism can be categorized into following types:

  1. Chain isomerism
  2. Position isomerism
  3. Functional isomerism
  4. Metamerism
  5. Tautomerism
  6. Ring – chain isomerism.
1. Chain isomerism

Compounds having same molecular formula and same functional group but different in their carbon chain are called chain isomers. The phenomenon of existence of such compounds is called chain isomerism. Examples:

chain isomerism

example of chain isomers

2. Position isomerism

The isomers having same carbon chains but different positions of functional group, substituent or multiple bonds ( i.e. double or triple bond) are called position isomers . Examples :

Position isomerism

3. Functional isomerism

Compounds having same molecular formula but different functional groups are called functional isomers. Phenomenon of existence of such compounds is called functional isomerism. Examples:

Functional isomerism

Note :  :  Functional isomers-

Alcohol ↔ Ether

Aldehyde ↔ Ketone

Carboxylic acid ↔ Ester

4. Metamerism

Compounds having same molecular formula but different in alkyl groups present in the either side of the same functional group are called metamers. The phenomenon is called metamerism. Example :

Metamerism

Note : ethers, ketones, 20 amine, 30 amine, esters etc. show metamerism.

5. Tautomerism

It is a special type of functional isomerism in which there is migration of α- hydrogen from carbon to electronegative atom and vice-versa with necessary rearrangement of covalent bonds. Eg . keto – enol tautomerism.

Tautomerism

  • In this example, there is migration of α- H from carbon to electronegative oxygen atom and vice-versa.
6. Ring – chain isomerism

In ring-chain isomerism, one of the isomers has an open-chain structure whereas the other has a ring structure.

  • They generally contain a different number of Π – bonds. Eg.

Ring – chain isomerism

Stereoisomerism (Space isomerism)

  • Compounds having same molecular formula but different arrangement of atoms or groups in space are called stereo isomers and the phenomenon of existence of such compounds is called stereoisomerism.
  • It is also known as space isomerism.

Stereoisomerism can be categorized into following types:

  1. Geometrical isomerism
  2. Optical isomerism
1. Geometrical isomerism
  • Stereoisomerism which arises due to different geometrical arrangement of atoms or group of atoms around rigid bond is called geometrical isomerism.
  • It is also known as cis-trans isomerism. Eg.

Geometrical isomerism

Note : If the similar groups are present on the same side of a rigid bond it is cis- form and if it is present on opposite side then it is trans form.

2. Optical isomerism
  • Optical isomers are compounds that are non- superimposable mirror images of each other.
  • Optical isomers have the same molecular formula but different spatial arrangement of the atoms.
  • Optical isomers are also known as enantiomers. Example :

Optical isomerism

Note:  Optical isomers (enantiomers) differ from each other in their optical activities.

If plane polarized light (PPL) is passed through solution of these substances, dextro (d-) enantiomers rotate the plane of polarized light to the right whereas laevo (l-) enantiomers rotate it to the left.

See the IUPAC nomenclature of Organic compounds ….

References